This invention relates to the materials used for and process for both the removal of sulfur compounds from a gaseous stream and for the water gas shift reaction. More specifically, the invention relates to the use of a nickel containing aluminate catalyst to provide for simultaneous desulfurization and water gas shift at temperatures of about 450° C.
The gaseous stream may originate from any partial oxidation or gasification process of a carbon containing feedstock. The gaseous stream may be a fuel gas originating from an IGCC (Integrated Gasification Combined Cycle) coal gasification plant, it may be a flue gas from a fluid catalytic cracking unit (FCC), it may be a synthesis gas (syngas) from steam reforming of natural gas, certain gasification reactions or from gasification of coal. Synthesis gas is the name generally given to a gaseous mixture principally comprising carbon monoxide and hydrogen, but also possibly containing carbon dioxide and minor amounts of methane and nitrogen.
Synthesis gas is used, or is potentially useful, as feedstock in a variety of large-scale chemical processes, for example: the production of methanol, the production of gasoline boiling range hydrocarbons by the Fischer-Tropsch process and the production of ammonia.
Processes for the production of synthesis gas are well known and generally comprise steam reforming, auto-thermal reforming, non-catalytic partial oxidation of light hydrocarbons or non-catalytic partial oxidation of any hydrocarbons. Of these methods, steam reforming is generally used to produce synthesis gas for conversion into ammonia or methanol. In such a process, molecules of hydrocarbons are broken down to produce a hydrogen-rich gas stream.
Regardless of the carbon source and gasification process, the generated fuel gas has to be substantially cleaned before being either burned in a gas turbine or used for chemical synthesis, e.g., methanol, ammonia, urea production, Fischer-Tropsch synthesis. The clean-up of hot fuel gases avoids the sensible heat loss due to the cooling and subsequent reheating associated with the wet scrubbing techniques using either chemical or physical solvents. Ideally, the clean-up of the fuel gas is done at the highest temperature that the fuel gas distribution system can be designed at. This could improve greatly the overall process efficiency, however, there are significant hurdles that need to be overcome before such a hot-fuel gas clean-up system is made commercially available. Only the hot particulate removal systems, i.e., candle filters or sintered metal filters, have been successfully demonstrated commercially for long term applications in a temperature range of 200° to 250° C. at the Nuon's Shell coal gasification plant in The Netherlands, and 370° to 430° C. in the E-Gas coal/coke gasification system at the Wabash River plant. All large scale warm desulfurization demonstration units have failed mostly due to inappropriate sulfur-scavenger materials.
Both large scale warm gas desulfurization units (Piñon Pine Air-Blown IGCC and Tampa Electric Polk Power station) used Zn-based S-scavenger materials. The Pifion Pine Air-Blown and Hot Gas Cleanup IGCC using a KRW air-blown pressurized fluidized-bed coal gasification system with Southern Utah bituminous coal containing 0.5-0.9% sulfur (design coal) and Eastern bituminous coal containing 2-3% sulfur (planned test). The purpose was to demonstrate air-blown, pressurized, fluidized-bed IGCC technology incorporating hot gas cleanup (HGCU); to evaluate a low-Btu gas combustion turbine; and to assess long-term reliability, availability, maintainability, and environmental performance at a scale sufficient to determine commercial potential. Steady state operation was not reached in the course of the 42 months demo operation and the Zn-based S-scavenger material failed since it did not hold up physically in the entrained bed reactor. Zn was lost during the 538° C. reaction via volatilization. The second large scale hot gas desulfurization demo unit at Tampa Electric Polk Power station intended to clean 10% of the fuel gas by a hot-gas cleanup system developed by GE Environmental Services, Inc. The hot gas desulfurization unit was an intermittently moving bed of Zn oxide based sorbent that operated at 482° C. The demonstration again failed due to very high attrition loss (which made operation with that particular sorbent far from cost effective) and due to significant reactivity loss because of Zn sulfate formation and Zn volatilization. (References: The Piñon Pine IGCC Project, U.S. DOE and Piñon Pine Power Project Reports, December 1996; January 2001 (DE-FC-21-92MC29309). The Tampa Electric IGCC Project, U.S. DOE and Tampa Electric Reports, October 1996; July 2000; August 2002 (DE-FC-21-91MC27363))
Some patents that disclose the use of Zn-containing S-sorbents include several assigned to Phillips Petroleum: U.S. Pat. No. 5,045,522; U.S. Pat. No. 5,130,288; U.S. Pat. No. 5,281,445; U.S. Pat. No. 5,306,685; and U.S. Pat. No. 6,479,429. There are also several patents assigned to RTI (Research Triangle Institute): U.S. Pat. No. 5,254,516; US 2004/0170549 A1; and U.S. Pat. No. 7,067,093. There is no prior disclosure of the simultaneous removal of S-compounds from a gaseous stream and the water gas shift reaction.
With the current state of development of hot gas cleanup systems, all the other contaminants besides the S-compounds and solid particulates can not be removed at equally high temperatures. Even more, due to the imminent CO2 regulations, all integrated gasification combined cycle (IGCC) gasifiers will have to be equipped with at least one CO-shift reactor, requiring thus cooling the fuel gas to temperatures adequate for the water gas shift catalytic reaction. In view of these CO2 regulations, the trend in the gasification industry is towards use of direct water quench gasifiers. The quench mode design significantly reduces the capital cost of syngas cooling, while heat integration maintains good overall thermal efficiency. The quench mode is advantageous for the water gas shift reaction as the raw syngas becomes saturated with steam generated by evaporation of a portion of the quench water. An entrained-flow slurry-fed gasification with direct water quenching is the preferred and commonly used option of GE Energy, and recently, Shell, Lurgi and Siemens also offer the water quenching cooling method. In addition to efficiently cooling the raw syngas and recovering part of the sensible heat, significant decontamination takes place in the quenching step. Solid particulates, alkali metals, non-volatile metals, chlorides, the bulk of metal carbonyls and part of ammonia are all removed in the water quenching step. The contaminants left in the raw syngas after the water quenching step include about 50-100 ppmv ammonia, 1 to 4 ppmv Ni and Fe carbonyls, about 50-100 ppmv HCN, Hg, As, and sulfur-containing gases, i.e., H2S and COS. All these contaminants must be removed before the syngas is either burned in a gas turbine or used for chemical synthesis.
This invention discloses a class of materials able to simultaneously completely desulfurize (remove H2S and COS) a fuel gas originating from a gasification process and shift the CO to CO2 in a temperature range of 250° to 550° C. The CO2 stream can be further shifted by adding an additional sweet CO-shift unit downstream of this integrated desulfurization and CO-shift unit. Thus, the hydrogen production is maximized and the clean, concentrated CO2 stream can be captured using either a physical solvent process, e.g., UOP's Selexol process, or alternatively using high temperature CO2 absorbents. This integrated desulfurization and CO-shift concept represents the next generation of synthesis gas treating. Currently, regenerable solvent-type acid gas removal processes are used in both IGCCs and chemical synthesis applications, e.g., UOP's Selexol process (U.S. Pat. No. 2,649,166 and U.S. Pat. No. 3,363,133) or Linde Engineering's Rectisol process (U.S. Pat. No. 2,863,5277). Unfortunately, these processes require cooling the fuel gas to low temperatures and then subsequently reheating it to temperatures adequate to its downstream use. This issue associated with the solvent scrubbing based clean-up processes can be addressed by using the concept disclosed in this invention. This invention relates to the use of a nickel containing aluminate catalyst to provide for simultaneous desulfurization and water gas shift at temperatures of about 450° C. The CO2 stream can be further concentrated (complete CO-shift) by adding an additional sweet CO-shift unit downstream of this integrated unit. Thus, the hydrogen production is maximized and the clean, concentrated CO2 stream can be captured using either a physical solvent process, or alternatively using high temperature CO2 absorbents. There are several main advantages associated with this concept: by continuously removing the H2S from the gaseous stream, the COS hydrolysis equilibrium is shifted completely to the right, the CO2 stream is concentrated via the water gas shift reaction, and also possibly the equipment costs could be greatly reduced.